The present invention relates to an ink which is particularly suitable for an ink jet printer of the kind referred to as the bubble type, in which expulsion of the droplets of ink is effected by the on-demand production of instantaneous vaporisation of a portion of ink in a nozzle. More particularly the invention relates to an ink which is essentially formed by an aqueous solution of a direct dye in a mixture of water and glycol wetting agents.
Various inks of the above-indicated kind are known, for example from U.S. Pat. No. 3,846,141 and U.S. Pat. No. 3,856,536, in which the dye is completely dissolved in the mixture which may also contain solubilising agents. Such inks which are used in a bubble-type ink jet printing element cause irregular expulsion of the drop due to electrothermal phenomena such as irregular absorption of current and over-boiling of a portion of ink. The printing is thus of poor quality, with dots which are not clear and which are in part missing.
U.S. Pat. No. 4,365,053 also discloses an ink in which the mixture includes a solvent formed by alcohol with a low boiling point, a soluble resinous component and water while the dye is formed by a white inorganic pigment with particles of a diameter up to 5 .mu.m, which is formed by titanium oxide which has a very high specific weight and which is in suspension in a resinous binder. The ink on the one hand is particularly suitable for a continuous ink jet printer in which a certain amount of ink is intended to circulate more than once through the nozzle, while on the other hand it is suitable for printing on dark carriers of plastics, glass or metal, in which the white pigment provides a sufficient contrast for reading purposes while the resinous component serves as a fixative. That ink is not suitable for printing on white paper, nor for on-demand printing devices, since in the absence of circulation of the ink, the pigment tends to form a sediment. In addition, due to evaporation of the water even upon short interruptions in operation of the printer, the resinous component suffers from encrustation and rapidly clogs the nozzles.
Finally, inks of the above-indicated type are known, for example from U.S. Pat. No. 4,373,954 and U.S. Pat. No. 4,536,776, in which a predetermined level of conductivity is achieved in order to attain the desired absorption of current in the ink, with a given control pulse. Those inks however do not ensure regular vaporisation of the ink to form the bubble so that the absorption of current in each pulse is liable to vary enormously.
In order to ensure a certain degree of regularity in the formation of the bubble, our published European patent application No. EP 0205243 also proposed providing for control of the printing element by means of a first pulse for heating the ink, followed by a vaporisation pulse. The heating pulse is controlled by a regulating circuit with a negative temperature constant, which reduces the effect of ambient temperature on the moment of initiating boiling. However, even with that control, the inks which are known hitherto do not make it possible to achieve a constant boiling temperature so that vaporisation and thus bubble formation are not regular, nor are they perfectly synchronised with the control pulse.